1. Field of the Invention
This invention relates to a control system for vehicular automatic transmissions, which is provided with a manual mode selector switch to prohibit operations in a highest gear position.
2. Description of the Prior Art
In a vehicular automatic transmission which is equipped with a fluid type torque converter and a transmission gear mechanism including a plural number of friction members for establishing a number of gear positions, the respective friction members are selectively engaged and released in various ways according to operating conditions of a vehicle to shift the transmission gear mechanism automatically into a speed position most suitable for the current operating condition of the vehicle. The operation of engaging and releasing these friction members is normally performed by a hydraulic control system incorporating a number of shift valves which are operated by opposing actions of a throttle pressure varying according to the extent of accelerator pedal depression or throttle opening rate and a governor pressure varying according to the vehicle speed, to select the gear position of the transmission gear mechanism on the basis of the contrastive relationship between the throttle and governor pressures, namely, between the extent of accelerator pedal depression and vehicle speed. Such a hydraulic control system for an automatic transmission is usually equipped with a manual shift valve thereby to switch the transmission manually to a desired speed range, setting a range over which the transmission is shiftable according to the above-mentioned opposing actions of the throttle and governor pressures, that is to say, setting the highest gear position to which the transmission is shiftable. The shift positions or selectable speed ranges of the automatic shift valves normally include forward drive ranges such as D-range, 2-range and L-range, and when the manual shift valve is in D-range position, the transmission gear mechanism is shiftable from a first speed position to a highest speed position of the transmission, normally to a third or fourth speed position.
For example, in the case of an automatic transmission with four forward speeds including "overdrive" as a fourth speed, the shift points of upshifts from third to fourth and downshifts from fourth to third are so determined as to permit shifts between the third and fourth speeds even in a throttle full open range for preventing overrunning of the engine and improving the performance quality at high speeds, for instance, at the time of the so-called 4-3 kickdown when gears are shifted from the fourth to the third speed position of a larger reduction ratio during operation in overdrive with a sharp increase in throttle opening rate due to a sudden depression of the accelerator pedal.
With a conventional hydraulic control system, the shift points between the third and fourth speeds are determined by the acting areas of throttle and governor pressures which are applied to a valve body of the 3-4 shift valve, a highest shift valve, and at the time of kickdown a detent pressure is applied to the valve body against the governor and throttle pressures. The governor pressure increases in proportion to the vehicle speed in low and middle speed ranges but it becomes dull in a high speed range due to oil leaks, so that at high speeds the shifts between the third and fourth speeds are effected by the throttle and detent pressures. Further, the hydraulic control system is provided with a solenoid valve for blocking a shift to the fourth speed of the highest gear position and forcibly effecting a downshift to the third speed during operation in the fourth speed, forcibly maintaining the third speed position in D-range operation when a manual mode selector switch which is provided beside the driver's seat is in a highest speed cancel position.
The conventional vehicular automatic transmission control system which permits to switch the operation to either the top speed non-cancel mode or the top speed cancel mode under any operating condition of a vehicle has a problem that overrunning of an engine is caused by a downshift when the O/D switch is inadvertently handled during operation at high speeds.